1. Field of the Invention
The invention provides for a method and apparatus for evaluating a membrane and, in particular, a method and apparatus for evaluating the integrity of porous membranes used in water treatment systems.
2. Description of the Related Art
Various methods are available for testing the integrity of a porous membrane. These methods include, for example, a traditional challenge test, where a sample of water containing microorganisms is passed through the membrane and the downstream product is analyzed for the presence and concentration of the microorganism. Other methods do not generally use actual microorganisms but measure some other physical characteristic of the membrane that in some way reflects the membrane""s ability to exclude the particles, solutes, or microorganisms of concern. These tests include the diffusion test, the bubble point test, the pressure hold test, turbidity measurements, particle counting, and conductivity tests.
In a standard diffusion test (DT), the membrane being tested is wetted with one fluid which is immiscible with a second fluid that is in contact with one side of the membrane. The pressure of the second fluid, typically air, is increased to a predetermined pressure or flow rate, generally one that has been recommended by the manufacturer of the membrane. At this pressure or flow rate, an amount of the second fluid will diffuse through the wetted membrane to the side of lower pressure. If the measured rate is the same as the rate suggested by the manufacturer, the membrane may be considered to be performing properly.
In the bubble point test (BPT), which is run in a similar manner to the diffusion test but at an increasing rate of pressure or flow of the second fluid, when the pressure of the second fluid reaches a critical level, the second fluid, typically air, may have expelled some of the wetting fluid from the pores of the membrane and may be visible as a series of bubbles on the low pressure side of the membrane. The lower the pressure required to produce visible bubbles, the greater the pore size or the size of a defect that may be present in the membrane. In addition to visually detecting the bubble point, the formation of bubbles may also be detected through the use of acoustic or optical measurements.
Measuring turbidity and counting particles have also been used to evaluate membranes. Both of these techniques provide general measurements of the amount of undissolved material that is passing through a membrane.
Another technique for measuring the amount of intrusion of a second fluid into a membrane is to measure the conductivity across a membrane. In this test, when a generally non-conductive membrane is intruded by a conductive fluid, the conductivity across the membrane increases and can be measured. From this measurement, the amount of fluid intrusion into the membrane can be indirectly determined.
The present invention is directed to a method and apparatus for evaluating a membrane. In one embodiment of the invention a fluid is supplied to the first side of a membrane at a pressure that is greater than the pressure on the second side of the membrane. The pressure on the second side of the membrane is an effective pressure greater than atmospheric pressure. The rate of transfer of the fluid from the first side of the membrane to the second side of the membrane is measured.
In another embodiment of the invention, a method for testing the integrity of a membrane is provided. A first side of a membrane is pressurized to a pressure greater than the pressure on the second side of the membrane and the pressure on the second side of the membrane is an effective pressure greater than atmospheric pressure. The amount of fluid transferred from the first side of the membrane to the second side of the membrane is measured and this amount is compared to the amount of fluid that would be predicted to be transferred by calculating the expected flow through a defect of a specific size.
In another embodiment, the present invention provides for a membrane testing apparatus. The apparatus includes a housing and a membrane mounted therein. The housing is divided into a first compartment and a second compartment. A fluid is contained in the first compartment at a pressure that is greater than the pressure in the second compartment and the pressure in the second compartment is an effective pressure greater than atmospheric pressure. A flow meter is in communication with the fluid.
In another embodiment, the invention provides for a membrane evaluation system that includes a wetted porous membrane having a first side and a second side. The pressure on the first side is greater than the pressure on the second side and the pressure on the second side is an effective pressure greater than atmospheric pressure. The system also allows for the measurement of the amount of fluid transferred from the first side to the second side.